The integrity and availability of cellular RNA is tightly regulated by a variety of RNA processing, turnover, and surveillance pathways. The eukaryotic exosome plays a major role in the processing and degradation of a wide variety of RNAs in both the nucleus and cytosol. Several protein factors associate with the exosome to identify appropriate RNA substrates and activate exosome activity. A major challenge is to understand the molecular details of how these proteins interact with RNA substrates and with each other to achieve substrate specificity. Notably, the only protein factor that is required for all known activities of the nuclear exosome i the RNA helicase, Mtr4. Given this central role in exosome activation, there is a critical need to understand how Mtr4 functions, how it interacts with RNA substrates, and how it mediates and is influenced by assembly of other exosome activating factors. Recent studies indicate that the arch domain of Mtr4 plays a previously unrecognized role in unwinding RNA substrates. The underlying basis for this observation is unclear, but is proposed to be a function of substrate interactions that connect the arch to the core helicase domains. The detailed interactions between Mtr4 and other complexes, such as the TRAMP complex, are also poorly understood. This project seeks to define the molecular basis for Mtr4 function by employing a variety of biochemical and structural approaches, combined with in vivo genetic analysis. The specific aims of this project are to (1) describe the role of the arch domain in RNA recognition and unwinding, and (2) characterize Mtr4 interactions with protein binding partners.